Process and apparatus for controlling metal etching operation



K. J. RADIMER PROCESS AND APPARATUS FOR CONTROLLING METAL ETCHINGOPERATION Filed Dec. 19, 1966 3 Sheets-Sheet 1 FIG. I.

'l .K' VEN TOR. KEN/V5 TH J: RAD/MEI A T TORNEV.

Oct. 28, 1969 K. J. RADIMER 3,

PROCESS AND APPARATUS FOR CONTROLLING METAL ETCHING OPERATION Filed Dec.19. 1966 3 Sheets-Sheet 2;

INVENTOR. KENNE TH J. RA DIMER A 7' TORNE' X Oct. 28, 1969 K. J. RADIMERI 3,475,242

PROCESS AND APPARATUS FOR CONTROLLING METAL ETCHING OPERATION Filed D90.19. 1966- v 3 Sheets-Sheet 3 FIGS United States Patent 3,475,242 PROCESSAND APPARATUS FOR CONTROLLING METAL ETCHING OPERATION Kenneth J.Radimer, Little Falls, N.J., assignor to FMC Corporation, New York,N.Y., a corporation of Delaware Continuation-impart of application Ser.No. 522,694, Jan. 24, 1966. This application Dec. 19, 1966, Ser.

Int. Cl. C23f 1/02 US. Cl. 156-49 15 Claims ABSTRACT OF THE DISCLOSUREThis application is a :continuation-in-part of my application, SerialNo. 522,694, filed Jan. 24, 1966.

This invention is directed to an improved method for conducting anetching process at a predetermined or preselected etch rate and, moreparticularly, it is concerned with the etching of copper foil mounted ona backing for the purpose of preparing so-called printed circuits foruse in electronic apparatus and to apparatus useful in said method.

My patent application Serial No. 522,694, discloses a process andapparatus for determining the rate at which metal is dissolved incommercial metal etching operations by measuring the rate at which ametal test article is dissolved in the metal etching bath. The rate ofmetal dissolution thus determined is utilized to control the addition ofactive etchant to the metal etching bath, to control a process variable,or to do both. Certain of the etching solutions widely used commerciallycontain two or more active components. Metal etching baths normallyshould contain an optimum concentration of active components. Duringoperation, these active components are consumed at different rates withthe result that replenishment of the difierent bath components isrequired at different and, usually, unpredictable rates.

Etching systems, such as the solution used in chemical milling, whichcontains ferric chloride and thiourea or a related compound, and suchmetal etching solutions as those containing mercuric chloride andammonium persulfate, disclosed in U.S. Patent 2,978,301, issued Apr. 4,1961, illustrate such complex systems.

The mercuric chloride-persulfate system is also complicated by thehydrolysis of the persulfate to degradation products, some of which areactive etchants. Each of these has a different etch rate at the samenormality. The etch rates in such systems are also affected by thehydrogen ion concentration and the concentration of catalysts which maypurposely be added, or which may be brought into the process duringoperation. It is advantageous separately to control the addition of eachof the active chemicals which affects the etching process. It is alsopreferable to be able to control the process variables separately.

It is an object of the present invention to provide a method forcontrolling the addition of one or more active components to a metaletching solution.

It is a further object of the present invention to provide a method forcontrolling the rate at which a metal dissolves in a metal etching bathby separately controlling the concentration, rate of addition of variouscomponents of the bath, and the process variables.

It is also an object of this invention to provide apparatus operative todevelop a basis for controlling the process variables.

Other objects and advantages of this invention will in part be obviousand will in part become apparent from the disclosure and drawingsherein.

In its broadest aspects, the present invention provides a method forcontrolling the etch rate of a metal etching bath by means of controlledaddition of at least one active agent in a metal etching bath containingtwo or more components that affect the etch rate, by first determiningthe difference in the rate of dissolution of a metal article in astandard metal etching solution being used for etching, and a secondidentical metal article which is etched with the same solutioncontaining an additional amount of one of the active agents beingcontrolled, or which has first been treated in a concentrated solutionof that active agent being controlled, before being dissolved in thesame etching solution as was used on the first article. If the secondmetal article dissolves at a different rate from the first, a need isindicated for addition of the active agent being controlled. An increasein this difference, other factors being constant, indicates a need for alarger addition of the active agent to the metal etching solution beingtested.

In more specific terms, reference should first be had to the followingequations identifying a persulfate etching bath:

These equations indicate first the usual hydrolysis of perdisulfate tothe bisulfate and permonosulfate ion.

The second equation indicates ionization or hydrolysis of this secondion to a bisulfate and hydrogen peroxide form.

The third equation indicates the decomposition of the hydrogen peroxideto water and oxygen.

Interestingly, each of the peroxygen components of this very complicatedmixture is an etchant and no one of them is of the same degree ofeffectiveness as the next under a given set of conditions. Thecomplexity of attempting to control etch rate of such baths on the basisof measuring the concentration of one or another component is such as tobe not only discouraging, but susceptible to such complications as to beundesirable. The problem of control of a solution or control of an etchrate in a solution of persulfate is best related to the overall rate ofetch without reference to the concentration of any particular one ofthese ions.

Frequently, mercuric chloride is used as an accelerator of thepersulfate etchant. It can be added to the persulfate solution, but theamount needed is very critical to maintain optimum etch rate in thebath. That is, optimum etch rate may be with a mercuric chlorideconcentration of the order of 3-5 parts per million, However, additionto a concentration beyond 5 parts per million will not necessarilyproduce a proportional increase in the rate of solution. Hence, optimumcontrol is best achieved in the overall etch by maintaining theconcentration of ingredients at a level on the low side of the maximumso that the addition of the mercuric chloride can produce a significantincrease on the etching rate. Similarly, presence of too much ammoniumpersulfate in the etchant can decrease etch rates. Hence, for controlpurposes, it is best to operate the etch bath at a rate of 510% below amaximum rate of etch so that the bath will be immediately sensitive toadditives.

In accordance with this invention, therefore, it is the purpose of themethod and of the technique to develop an overall etch rate conrtollerbased upon measuring the point of actual physical disappearance of asample of the material being etched at a given level or depth in thesolution by physically inspecting the material being etched at thedesired level and scanning it over a predetermined distance as itapproaches that level, in order to develop a range within which controlcan be achieved. My method, therefore, consists of simultaneouslyfeeding at least two tapes carrying copper, or other metal to be etched,the said tapes being identical to each other and of the same metal asthat being etched, into an etching bath, such as a persulfate bath,which is catalyzed with mercuric chloride to a concentration which is anappropriate optimum level, but on the low side of optimum. Since the twotapes are physically identical, they should etch at the same rate andthe disappearance of copper should be detectable at a given point in thesolution corresponding to the level reached by the tape at that point,based upon the rate of consumption of copper in the etch bath. One tapeis fed directly into the solution, and the second tape is first fedthrough a mercuric chloride solution, so as to develop on its surface aneffective small incremental concentration of mercury, so as to cause thetape to be etched more rapidly when it enters the etching solution withthe first tape. Under these circumstances, the two tapes travelling atthe same rate will have different levels of copper disappearance as theytravel through the solution, and the difference in levels can be used asa measure of a need for mercuric chloride in the etching solution, themeasurement being used as a signal to effect an addition of mercuricchloride to the solution and, under the circumstances where thedisappearance of the copper is substantially at the same level, at adeep point in the solution, it can be used as a measure of the need forthe addition of persulfate. The actual rates of etching of the tapes maynot be identical with etching rates in the bath being controlled becauseof differences in the mode of application of etchant to the copper tapeand to the printed circuit board in the etching bath, so that the methodis actually one for controlling a bath to produce a pre-selected etchrate in the etching bath rather than an effort simply to measure therate of etch on any particular surface of copper in the solution. It isof course obvious that the device described can also be used to measureetch rates on the copper tapes.

The invention, accordingly, is based upon this method of developing anoverall control of etch of a copper foil in a given etching bath at apreselected rate, by comparing a copper foil sample etched in theetchant being controlled with a sample having an incrementally largermercury activity, and utilizing the difference as a measure of the needfor additional mercuric chloride catalyst,

The apparatus described herein in connection with the accompanyingdrawings provides a means for practicing the method of this invention,in which FIGURE 1 is a phantom view of the path of the two tapes throughthe apparatus;

FIGURE 2 is a right side elevation of the apparatus, partially insection; and

FIGURE 3 is a front elevation of FIGURE 2, also partially in section.

The application of the basic process of the invention is illustrated inthe control of a metal etching bath using a tape in which a metal foilis laminated to a flexible nonmetallic tape. A first such metal tape istreated in a concentrated solution of the first active agent and thenetched in a solution taken from the commercial metal etching bath beingcontrolled. The test metal etching baths should preferably be maintainedat the temperature used in the etching machine associated with thecontrol device. The time necessary to dissolve the pre-dipped metal foilon the first tape is determined. Similarly, a second standard metal tapeis treated with the same metal etching solution but without having beenpre-dipped and the time necessary to dissolve the metal foil on thesecond tape is determined. The additional increment of time necessary todissolve the second strip when compared with the time necessary todissolve the first test strip is determined. During operation, anincrease in this time increment is indicative of a need for addition ofthe first active agent.

The following specific example of the process illustrates thisinvention.

A copper foil (one ounce per square foot being 0.0014 inch thick)laminated to a transparent polyester tape backing, hereinafter referredto as the copper-tape, was predipped by passing it through a solution ofmercuric chloride containing 25 parts per million of mercury at a ratesuch that its residence time was one minute and then etched in a onemolar ammonium persulfate standard solution. The copper dissolved inthree minutes and 13 seconds when the standard solution contained 5parts per million of dissolved mercury added as mercuric chloride; threeminutes and 25 seconds when the standard solution contained three partsper million of mercury; and four minutes and 37 seconds when thestandard solution did not contain any dissolved mercury. Standardcopper-tape which had not been pre-dipped in the concentrated mercuricchloride solution was also dissolved in the same standard etching baths.The bath containing 5 parts per million of dissolved mercury requiredthree minutes and 45 seconds to dissolve the copper; the bath containingthree parts per million of dissolved mercury required four minutes andten seconds to dissolve the copper; and the bath which did not containdissolved mercury required 11 minutes and 15 seconds to dissolve thecopper. The foregoing information is utilized in a system forcontrolling the concentration of mercuric chloride at a level of 5 partsper million in a commercial metal etching bath containing about 1 molarammonium persulfate solution.

Two test copper-tapes, side by side, are moved into a portion of 1 molarammonium persulfate metal etching bath being tested, preferably at aconstant speed. Control of the dissolved mercury concentration at 5parts per million was desired. One of the test copper-tapes was firstpassed through a mercuric chloride solution containing 25 parts permillion of mercury for a period of 1 minute. While immersed in the testsolution, each of the coppertapes is monitored by a sensor which movesalong the copper-tape, preferably in the direction in which thecopper-tape is moving, and determines the time necessary to dissolve thesubject copper-tape. The sensor for the predipped copper-tape had beenadjusted to lag behind the sensor for the untreated copper-tape by adistance corresponding to an etching time difference of 32 seconds. Whenthe bath being tested contains 5 parts per million of dissolved mercury,both sensors are actuated at the same instant. When the concentration inthe test bath drops below 5 parts per million of dissolved mercury, thepre-dipped copper-tapes sensor is actuated before the other sensor.Thus, when the bath contained three parts per million of mercury, theadditional increment required in etching time would be 13 seconds. Ifthe bath contained no dissolved mercury, the additional increment wouldbe 6 minutes and 6 seconds. Where the sensors determine a larger timeincrement than that determined from the standard solutions, theinformation is utilized to add mercuric chloride to the etching bathtested. This may be accomplished by using the action of the predippedtapes sensor to open a solenoid valve and the action of the other sensorto close it. The valve may be controlled to introduce a constant smallflow rate of mercuric chloride solution into the commercial metaletching bath. This may be repeated in each sensing cycle with the resultthat the concentration of mercuric chloride would be adjusted upwarduntil the desired concentration was attained. Additions of mercuricchloride during each of a number of consecutive cycles might berequired.

More complex control systems utilize the determination of the differenceof time necessary to dissolve the untreated tape when compared with thetime necessary to dissolve the pre-dipped tape, to determine the actualconcentration of dissolved mercury in the bath. This could then controladdition of the necessary quantity of mercuric chloride to attain theoptimum production concentration.

The mercury concentration is adjusted as already described. Therefore,an optimum concentration of mercuric chloride catalyst will bemaintained in the etching bath. With such an optimum mercuric chloridecatalyst concentration present, it is possible to know of to determinethe maximum etching rate which one can obtain with any given etchingequipment being used with persulfate etchant. The apparatus which is thesubject of this invention can be used to add persulfate as required toproduce the desired etch rate which, most desirably, is selected to beonly slightly less than the maximum etch rate obtainable with catalyzedpersulfate in the given etching system. One of the two copper bodiesbeing etched in this device is monitored at a predetermined level forthe presence of copper at a level below the free-surface of the etchantor beyond the point at which the etchant is first applied to the bodybeing etched. The predetermined level is calculated by the rate oftransport of the tape multiplied by the thickness of the copper beingetched away, divided by the rate of etching at the desired catalystconcentration. The predetermined level may also be determined byexperimentation with the test device. If copper on a tape has beenetched away at this pre-selected level, no additional persulfate isrequired. If the copper is still present at that level, additionalpersulfate can be injected into the etchant.

The method of the present invention for controlling the etch rate of ametal etching bath may be simplified in operation by deter-miningconditions under which the metal tape which first passes through theconcentrated solution dissolves at almost or exactly the same rate asthe metal tape which passes through the metal etching bath controlled atthe desired concentration and rate of etching. This may be accomplishedby adjusting the concentrated solution so that it results in an etchingrate the same as, or only slightly greater than, that of the etchingrate desired in the metal etching bath. In operation, slower dissolutionof the metal tape etched in the metal etching bath being controlled isdirectly indicative of the need for addition of etching agent. Ineifect, this variant of the invention utilizes a zero predetermineddifference in the metal etching rate of the desired metal etching bath,and the more concentrated solution.

In another variant of the method, the control of the etch rate may besimplified by predetermining a rate of addition of the etching agentbeing controlled, to maintain the etching rate in the metal etching bathat, or just below, the desired metal etching rate. During operation,slower dissolution of the metal tape in the metal etching bath whencompared to the tape first passed through the concentrated solution ofetching agent is indicative of the need for more rapid addition of theetching agent. :In this variant of the invention, the decline in etchingrate that would be expected because of any predicted continuous loss ofetching agent from the bath is largely, but not completely, eliminatedby adding etching agent at a rate almost sufficient to compensate forthe expected loss. The addition of the necessary remaining etching agentto control the system, is accomplished by the apparatus and process ofthis invention.

The method of the invention is directly applicable to spray etchtechniques. The point of entry of the tape into the spray can be treatedin the same fashion as the point of entry of the tape through the freesurface of an etch solution.

A suitable apparatus for practicing the process of the present inventionis illustrated in the drawings herein.

The apparatus illustrated is particularly adapted for the control of oneor two active etchant reagents in a complex system, e.g., the control ofmercuric chloride and if desired, the control of persulfate, in thesystem described hereinbefore. The path of the two copper-tapes 10 and11 through the apparatus is depicted in FIGURE 1. Each of thecopper-tapes is identical, being a thin copper foil fixed on a flexiblestrip of transparent polyester backing.

Copper-tape 10 is passed from supply reel 12 directly into reservoirtank 16 which contains etching solution. Copper-tape 11 is taken fromsupply reel 13 and passed into a pre-dip section comprising tank 14which contains a solution of the active etchant being monitored, e.g.,mercuric chloride, and is then rinsed in tank 15. Tape 11 is then passedinto and through reservoir tank 16.

The path of copper-tapes 10 and 11 through reservoir tank 16 areidentical. The etching conditions are identical. The thickness of copperon the copper-tape, and the distance travelled through the etchingsolution are correlated with the rate of travel of the tapes so that allthe copper be dissolved before the tape reaches guide rods 17.

The point at which the copper is dissolved from each of saidcopper-tapes is determined by the sensing apparatus which carries alight source-photocell combination for scanning each of thecopper-tapes. Copper-tape 10 is scanned by light source 23 and photocell24. Copper-tape 11 is scanned by light source 25 and photocell 26. Theselight source-photocell combinations ascend and descend at a rate fasterthan the descending copper-tapes. The light source-photocell combinationare activated only while descending. The respective lightsource-photocell combinations determine the point at which the copper isdissolved from the copper-tape which it is scanning.

The tapes after dissolution of the copper, are passed around guide rods17 and then around guide rods 27. The now upwardly ascending tapes 10and 11, are pulled by driven take-up reels 28 and 29. After passagethrough reservoir tank 16, they are passed between capstans 30 andspring loaded guide rollers 31, and then taken up on reels 28 and 29.

The preferred apparatus is illustrated in detail in FIG- URES 2 and 3.As depicted therein, copper-tape 10 is passed directly to reservoir tank16 from tape supply reel 12. Copper-tape 11 is taken from tape supplyreel 13, passed over guide rod 18, and through the mercuric chloridesolution in tank 14 around guide roller 19. Coppertape 11 is then passedover guide rod 32, through rinse tank 15 around guide roller 33, andthen positioned by guide rods 34 and passed into reservoir tank 16.

The portions of the apparatus which are in contact with the corrosiveetching solution, and particularly those immersed in reservoir tank 16,are constructed of corrosion resistant material such as polymethylmethacrylate, and preferably utilize stainless steel fasteners when theetchant is ammonium persulfate.

'Each of the light source-photocell combinations is supported on a panelwhich is controlled to ascend and descend in reservoir tank 16 in thespace between the descending and ascending paths of tapes 10 and 11,Photocell 24 is supported on panel 40. Light source 23 is connected topanel 40 by link 41. In a similar fashion, photocell 26 is supported onpanel 42. Light source 25 is connected to panel 42 by link 43. Panels 40and 42 are secured to each other by adjusting screw 44. In certainoperations, particularly to avoid corrosive attack, the light-sourcephotocell combinations may be mounted outside of the tank. The lightsources would be on one side of the tank, positioned to illuminate lightpaths through front transparent windows positioned in line with thepaths of the tapes being scanned and then through transparent rearwindows to the externally mounted aligned photocells.

Panel 40 is supported in its ascending and descending traverse bytubular arm 45. Panel 42 is similarly supported by tubular arm 46. Bothof these tubular arms are aflixed to connecting link 47. Electricalconnections to the light sources and from the photocells on the panels,are passed through the tubulars arms 45 and 46, and then to outsideelectrical connections through wires 48.

Tubular arms 45 and 46 are controlled in their ascending descendingmovement by attachment of link 47 to chain 50 which rides on lowersprocket 51 and upper sprocket 52. Control of the sensing apparatus sothat it is activated only when descending is achieved by contact ofrollers 53 with contact switch 54.

Power for the movement of the tape take-up reels 28 and 29, and also forthe movement of chain 50 is supplied through shaft 55, which issupported by bearings 56. Shaft 55 drives sprocket 51. It also drivesthe tape take-up reels 28 and 29 through slipping elastic drives 57 and58 respectively. Shaft 55 is driven from sprocket 59 by means of chaindrive 60, which in turn is powered by a conventional motor and reductiongear to provide the desired speed.

Additional units may be used for etchant components being controlled inaddition to the two components which may be controlled by theaforedescribed apparatus, and for process variables being controlled.Such additional units could be constructed in tandem operating from thesame motor shaft.

In another variant of the method, the pre-dip section may be avoided.Instead, each of the copper-tapes are etched in separate reservoir tanksunder identical conditions. The solution in one tank will be a portionof the metal etching bath being controlled. The solution in the othertank will be the same solution, to which has been added an incrementalamount of one active etching agent.

In the preferred method of this invention, with the test metal strip inthe form of a continuous strand, e.g., a wire and preferably a foil,measurement of the rate of dissolution of the metal utilizes transmittedlight and optical sensors which move along the strip as aforedescribed.Other means for measuring the rate of dissolution may be utilized, suchas ahe thickness of the strip, the ability to absorb other radiations,such as X-rays, gamma rays, etc., electrical resistivity, detectingsystem-s depending on capacitance of a conducting plate in proximitywith the strip and other light radiation systems depending uponreflectance of impinged light rather than transmission of such light.

The preferred sensing device of the present invention utilizes a lightsource and a light pickup travelling along a tape and determining thepoint at which the tape is entirely dissolved. Equivalent systems may beused which do not involve a moving sensor, or measure a point of metaldissolution less than complete dissolution. Thus, a plurality ofphotocells positioned along the tape could be utilized to determine thetime necessary to dissolve the tape. The photocells are connected to acommutator. A brush riding on the commutator could sense the timenecessary to dissolve the metal. Using two such devices, one for thepre-dipped tape, and the other for the untreated tape, the addition ofthe requisite active agent into the etching bath could be controlled.Another system utilizes a wholly electrical system rather than a lightsystem in which an AC potential is impressed between the tape and anumber of sensing electrodes positioned along the tape. Where suchelectrodes are parts of small conductivity cells, the conductance of thecell consisting of the copper on the tape and each small electrode willbe comparatively large if copper on the tape remains unetched near agiven small electrode, while the conductance of the cell consisting ofan electrode located near that portion of the tape from which the copperhas been etched will be comparatively small. A suitable circuit canutilize this information in a manner similar to that describedhereinbefore.

What is claimed is:

1. The method for controlling the etching rate of a metal etching bathcontaining at least two active etching agents by the controlled additionof at least a first etching comprising comparing the rate at which afirst metal article dissolves in the metal etching bath beingcontrolled, with the rate that an identical second metal articledissolves in an etching solution that differs from said metal etchingbath by containing a higher concentraction of said first etching agentat the surface of said second metal article, whereby when said firstmetal article dissolves at a rate different from that of said secondmetal article, an addition of said first etching agent is made to changethe etching rate.

2. The method of claim 1 comprising (i) treating a first metal articlein a concentrated solution containing an amount of a first etching agentin excess of the amount desired in the metal etching bath, and thencontacting said first article with a metal etching bath containing thedesired concentration of the bath components, and measuring the rate ofdissolution of said first metal article;

(ii) contacting a second metal article with said metal etching bath andmeasuring the rate of dissolution of said second metal article;

(iii) determining the difference in the rates of dissolution of said twometal articles;

(iv) treating a first test metal article with said concentratedsoltuion, and then contacting said first test metal article with themetal etching bath being controlled and measuring the rate ofdissolution of said first test metal articles;

(v) contacting a second test metal article with said metal etching bathbeing controlled and measuring the rate of dissolution of said secondtest metal article and (vi) determining the difference in the rates ofdissolution of said two test metal articles, and then comparing saiddifference with that determined in step No. (iii) hereof, whereby whenthis second difference is larger than said first difference, an additionof said first etching agent is required to increase the etching rate.

3. The method of claim 2 wherein said metal articles are metal tapes.

4. The method of claim 3 wherein the difference in rates at which themetal tapes dissolve is determined by measuring the time required tocompletely dissolve the metal of each of said tapes.

5. The method of claim 4 wherein each of said tapes is a copper foillaminated to a flexible plastic film backmg.

6. The method of claim 5 wherein said metal etching bath is an aqueouspersulfate-mercuric chloride etching bath.

7. The method of claim 6 wherein said method is used to control theetching rate by control of the amount of mercuric chloride etchant addedto said metal etching bath.

8. The method of claim 7 wherein the addition of persulfate etchant tosaid metal etching bath is controlled by determining whether the copperhas been completely dissolved from the tape after the elapse of apredetermined amount of time.

9. Apparatus for controlling the etching rate of a metal etching bath bycontrolling the concentration of an etching agent in said metal etchingbath comprising a container having at least two compartments, one forcontacting a solution containing said etching agent with a metal strip,and a second for contacting a portion of the metal etching 'bath beingcontrolled with at least two metal strips,

means for contacting a metal strip with said solution and thencontacting said metal strip with said metal etching bath,

means for contacting a second metal strip with said metal etching bathunder the same conditions as those for contacting said first metal stripwith said bath, and

means for measuring the rate that metal is dissolved from each of saidmetal strips as a result of contact with said metal etching bath. 10.The apparatus of claim 9 containing means for comparing the differencein the rates at which metal is dissolved from each of said strips andcontrolling the addition of said etching agent to said metal etchingbath in response to the difference in said rates.

11. The apparatus of claim 9 containing means for passing a metal tapeon a plastic hacking into a tank containing said metal etching bath;

means for passing an identical meta-l tape on a plastic backing througha tank containing a concentrated solution of said etching agent and thenpassing said tape into a tank containing the metal etching bath;

means for passing said two metal tapes in side by side relationshipthrough said metal etching bath, and

means for determining the point along the path of travel of each of saidtapes through said metal etching bath at which the metal is dissolvedfrom said plastic backing.

12. The apparatus of claim 11 wherein said means for determining each ofsaid points at which said metal is dissolved, is the combination of alight source directed across the path of said tape and at least onephotoelectric cell positioned at the opposite side of said tape.

- metal etching bath.

References Cited UNITED STATES PATENTS 2,762,036 9/1956 Triman 15672,933,094 4/1960 Cunniff et a1. 134-57 X 2,978,301 4/1961 Margulies etal. 15618 X 3,013,569 12/1961 Sterczala 15618 3,032,753 5/1962 Knapp etal. 156345 X 3,300,362 1/1967 Crosfield 156345 HAROLD ANSH-ER, PrimaryExaminer JOSEPH C. GIL, Assistant Examiner US. Cl. X.R.

